A lot of us think that Rajdeep is brought to gain sympathy for Nandini. But, on the contrary, he is brought to bring Kunal-Nandini closer.
Every time Rajdeep has gone to extreme level of violence, Kunal has come to Nandini's rescue, which brings them closer. Everytime Kunal has his attention divided, Rajdeep comes into picture to bring back his attention to Nandini only.
Rajdeep when he tried to humiliate Nandini in society- To bring Kunal to Nandini's defense and give Nandini a good idea about Kunal & also push Nandini to think more about Kunal. At that time, Kunal already had feelings for her.
Rajdeep when he tried to rape Nandini- To bring Kunal to Nandini's home & break Mauli-Kunal conversation.
Rajdeep on Mauli's brother wedding day- Led to KuNan consummation.
Rajdeep on Nandini's wardrobe malfunction- To bring Kunal as savior & push Kunal to extreme defense of Nandini by beating him up.
Rajdeep to beat Kunal after revelation- To make Nandini take stand for Kunal.
Rajdeep during humiliating Nandini by mahila morcha- To make Kunal speak to Nandini after revelation since Kunal went completely quiet after that & push himself to protect her once again by forcing Rajdeep to sign divorce papers.
Rajdeep now during kidnapping track- To bring Nandini-Kunal to talk again since Nandini doesn't want to talk to him & clear out the thing about Mauli's gift from Nandini on her b'day.
Everytime Rajdeep has come, Kunal has been pushed more towards Nandini & Nandini towards Kunal.
So, if again in future Kunal-Nandini have a fight or distance, Rajdeep will be brought to bring the divine lovers close. Apart from divinity, Rajdeep's evilness too is bringing the star-crossed lovers close.
P.S- It appears Rajdeep gets Nandini ready as a bride with mangalsutra & sindoor & leaves her to burn in fire. But Kunal saves her on time & brings her home in the same attire & then they have conversation about Nandini's behavior to go away from Kunal. So, as per OLVs, no one has gotten married. So every spoiler till now, stands kind of false.